1. Field of the Invention
The present invention relates to an electrophoresis chip that is used for separating a sample such as proteins in a channel by electrophoresis and then carrying out a process to detect the sample.
2. Description of the Related Art
The following system was recently proposed. In this system, a microfluidic chip that adopts a configuration in which a lid that covers the upper surface of a channel can be detached is used to first separate a solute of, for example, proteins contained in a sample solution in the channel by capillary-electrophoresis in a state in which the lid hermetically seals the channel. An ionization accelerator referred to as a matrix for assisting the ionization of the solute of protein is next added in a state in which the lid is detached and a matrix-assisted laser desorption/ionization mass spectrometer (MALDI-MS) is next used to ionize the solute by scanning with a laser along the channel on a microfluidic chip and then to detect the solute by mass spectrometry.
Systems for detecting by mass spectrometry are also proposed in the papers by K. Tseng et. al in SPIE (Vol. 3606 (1999) pp. 137-148), J. Liu et. al in Analytical Chemistry (Vol. 73 (2001), pp. 2147-2151), and M. Mok et. al in Analyst (Vol. 129 (2004), pp. 109-111), as well as in the documents JP-A-2003-071263 and JP-A-2005-124332. In these documents, systems are proposed that use a construction having an open upper surface formed on the surface of a microfluidic chip, i.e., a groove-shaped channel that lacks a lid structure for covering the channel, to separate a solute of, for example, proteins included in a sample solution in the channel using capillary-electrophoresis, and then use a matrix-assisted laser desorption/ionization mass spectrometer to ionize the solute by scanning the solute with a laser along the channel on the microfluidic chip and detect it by mass spectrometry. In these systems, the solute that has been separated in the channel is dried so as not to disrupt the separated state, a solution in which an ionization accelerator referred to as a “matrix” has been dissolved is added to the separated and dried solute to produce a matrix crystal in a state in which the sample is mixed, following which mass spectrometry is carried out.
In the systems described above, however, when the detected proteins are identified by using peptide mass fingerprinting (PMF), a process is necessary for adding a solution that contains a digestive enzyme such as trypsin to the protein while taking care not to disturb the positions of the proteins that have been separated in the channel to digest and break down in the peptides. The original proteins can be identified by subjecting these peptides to mass spectrometry. In this digestion process, in contrast to operations that are completed instantaneously such as when adding the matrix solution, the solution state must be maintained for at least several minutes. As a result, keeping the position of the proteins undisturbed in the digestion process is problematic in the case of the ordinary channel construction described in the documents mentioned hereinabove. In other words, a special channel is required that functions in specific cases as a flow path for electrophoresis to achieve a state in which the liquid is continuously contiguous in the channel while in other specific cases the special channel functions as wells in which droplets to not mix.
As such a channel, a construction can be considered in which theoretically, as shown in FIG. 1, channel 102 of a two-step excavated construction is formed that creates steps in the direction of depth of chip substrate 101. Such a construction can function as a flow path when there is a large volume of solution, but when there is a small volume of solution, portions 103 that are one level deeper can function as wells to prevent mixture of droplets of solution between the wells.
However, when a channel of this construction is actually produced, the amount of liquid that is handled on a microfluidic chip is extremely small, and the effect of gravity is therefore limited and the effect of hydrophilic or hydrophobic properties greater than that of gravity. Accordingly, maintaining solution independently in each well is problematic when only the depth of the channel is increased, and filling wells with solution is also problematic.